support for empty requests

In case of empty POST request, just emit 'end' rather than
complaing on possible missing Content-Type header
test cases on empty post requests
attach callback before parsing headers

In case callback function is given to form.parse(), it should be
attached to formidable events prior to writeHeaders()
Update test-incoming-from.js test case accordingly.
Merge pull request #194 from egirshov/master

Handle empty requests